Not Applicable.
Not Applicable.
The present invention relates to oxygen sensors and particularly to oxygen sensors based on superionic conduction between electrodes attached to a solid body formed of an oxide compound, such as yttria stabilized zirconia. Sensors of this general construction may be operated as pumps, to move oxygen through the material which is generally set up as a barrier between a reference environment and an environment to be measured. In this case, the electrodes are energized to induce a current of oxygen ions migrating through the body. They may also be operated as Nernst cells, i.e., batteries in which a potential is induced by the difference in oxygen partial pressures on both sides of the electroded media. In either mode of operation, the electrodes are generally placed across a suitable circuit, and the level of current flow achieved, or the magnitude of the induced potential difference, respectively, provides a measure of oxygen concentration in the surrounding environment. In some practical devices, both forms of operation may be applied: some amount of pumping may be employed to verify, establish, or correct a reference level, followed by reading of a potential difference induced across the wall by the pressure of a sample to be measured. A previously-established formula is then applied to convert the electrical signal to a partial pressure reading.
Typical sensor constructions utilize porous electrodes of platinum or gold. When operated as a pump, oxygen contacting the cathodic surface acquires an electron, initiating ionic migration in the applied electric field gradient to the anode, where it gives up the electron and oxygen is released, passing out through the anode. Ion mobility generally requires that the doped oxide be operated at elevated temperatures, above several hundred degrees Celsius, and up to about 1100 degrees. When operated as a pump, the mechanisms of ion movement in the applied field may be affected by numerous factors, including temperature, thickness, electrode area, and the presence of non-equilibrium conditions within the material from its previous operation, so that the attainment of a repeatable condition may be difficult even when the wall thickness is quite small. These devices usually incorporate a diffusion barrier and pump against a diffusion-limited flow in order to achieve a meaningful measurement. In operation as a Nernst cell, the difference in oxygen pressure results in a higher oxygen/oxide equilibrium point at one surface, and a consequent polarization within the solid material, so a potential appears across the wall. This too depends upon a number of factors, and is affected by the presence on non-equilibrium processes if sufficient settling time is not allowed.
In one form of sensor, a bulk oxide material is shaped into a sensing body with a surface configuration, typically a sheet or tube, that defines a barrier between a reference environment or closed reference cell, and the measurement environment to be tested. Porous platinum electrodes are generally placed on two opposed surfaces the sensor, which operated at a temperature of 500-1200xc2x0 C. to permit a sufficient level of ionic mobility. After an initial calibration, the Nernst voltage may be read to provide a measure of the oxygen level in the test environment. The reference cell may be periodically replenished by pumping in a reference environment. When operated as a pump, oxygen passing through the porous cathode electrode acquires a charge, and initiates an ionic current flow through the oxide material of the wall toward the anode in the applied gradient. Upon reaching the opposed electrode, ionic oxygen is neutralized and released as gaseous oxygen. These sensors are quite durable; some may be baked out or recalibrated periodically, and may be calibrated in the field by various techniques typically involving the provision of a reference gas on one side of the sensing body. However, the fundamental mechanism of ion flow through a relatively thick body, in dependence upon the oxygen pressure difference existing across that body, results in memory effects and a sensor with relatively slow response and relaxation times.
Another form of a solid superionic oxygen sensor uses doped zirconia or a similar substance in the form of a thin film deposited on a suitable substrate, typically a ceramic, when designed as a pump a diffusion barrier may be incorporated to define the response of the device. These thin film sensors may have shorter ion travel paths, and because of their thin film structure, the electric field gradient between electrodes may be higher and their response and settling times may be shorter. They can also operate at somewhat lower temperatures, e.g., about 350xc2x0 C., and may be adapted to different operating ranges, but must be somewhat protected form contaminants in the environment, and protected from excessive flow. However, such diffusion barriers may be vulnerable to aging, making sensor parameters and performance unstable over time.
Both kinds of sensors have the drawback that when a background gas such as hydrogen or certain hydrocarbons are present in the mixture being analyzed at low oxygen concentrations, the background gas may result in a faulty reading. This cross-sensitivity problem is particularly severe when the oxygen level is in the ppb range and there is a trace quantity or more of hydrogen. Such conditions may frequently arise in various semiconductor fabrication processes that require monitoring of low oxygen levels. The nature of this interfering effect may possibly involve a hydrogen diffusion current, or may involve electrode-catalyzed burning of the background gas with the oxygen that is present. Whatever its mechanism, the presence of trace hydrogen has been found to defeat or substantially impair the measurement of oxygen concentrations in the low range by these solid oxygen sensors.
Accordingly, it would be desirable to provide an improved oxygen sensor.
It would also be desirable to provide an oxygen sensor having ppb sensitivity.
It would also be desirable to provide an oxygen sensor having ppb sensitivity effective in the presence of hydrogen or background gas.
It would also be desirable to provide an oxygen sensor having reduced cross-sensitivity.
It would also be desirable to provide such a sensor that is fabricated using thin film technology and may be manufactured in quantity and deployed as an electronic chip.
One or more of these and other desirable features are achieved in accordance with the present invention by an oxygen sensor in which a sensing body formed of a crystalline material, such as zirconia, has electrodes of first and second polarities disposed on a single surface thereof. The electrodes are arranged to induce superionic oxygen transport in the body along current paths extending substantially parallel to the surface at a shallow depth in the crystalline material. The electrodes may be interdigitated or otherwise juxtaposed anodic and cathodic electrodes that define a surface pattern of ion conduction areas for sensing the level of oxygen at the surface. The body of the sensor may be formed as a thin film of polycrystalline material, which is, for example sputtered onto a suitable substrate, but is preferably constructed from a thin slab of single-crystal material. The provision of electrodes on a single side of the device limits the effective ionic conduction to a thin-film region of the electroded surface, and response is governed by electrode area and inter-electrode spacing rather than diffusion properties of a structural wall or a coated barrier. The electrodes are preferably poisoned to inhibit catalytic burning effects. The thin film device so formed operates at a relatively low temperature in the range of 250-400xc2x0 C., preferably about 300-350xc2x0 C., with negligible leakage current, and allows measurements of oxygen concentration to be accurately performed in the low ppb range.
The oxygen sensor may be fabricated together with a heater, such as a resistive heating/temperature sensing element deposited on the back face of the crystal, or may be attached to a heater chip to provide a small and integrated unit of versatile construction. The heater chip preferably includes both a heating electrode, and a resistive thermal device that may, for example, provide a feedback or control signal for controlling heater temperature in the desired range. Multiple sensors may be formed lithographically in a large array with suitable lead pads, then scored, separated into single units, and mounted on a heater chip to provide a self-contained integrated unit of versatile application.